Generally speaking, the power supply stage of an audio amplifier must accomplish two distinct goals. First the power supply stage must receive DC input voltage and step that input voltage up to a higher output voltage which is capable of generating additional output power. Second, the power stage must provide bipolar (i.e. positive and negative) rails for use by other amplifier components in amplifying an input audio signal from a relatively low volume to a higher volume.
Most power stages step voltage up from a low voltage level to a high voltage level using a power transformer. A transformer is a device that raises or lowers the voltage of an alternating current. In its simplest form, a transformer is a static electric device consisting of two or more windings, which are mutually coupled. Generally, the winding which is electrically excited is referred to as a primary winding while the winding which is magnetically excited through coupling action is referred to as a secondary winding. By passing alternating current through the primary winding, a magnetic flux is generated which in turn generates a current through, and a voltage across, the secondary winding. By providing the secondary winding with more windings than the primary winding, a relatively low voltage can be increased to a higher value at the output of the transformer.
For the purposes of understanding the present invention, it is important that there are at least two different types of transformers, an isolated primary and secondary transformer and a non-isolated primary and secondary transformer, each of which has advantages and disadvantages. An isolated transformer includes separate primary and secondary electrical circuits which are magnetically coupled via the transformer core. Hence, a primary winding wraps around a portion of the core and a separate secondary winding also wraps a portion of the core. The primary and secondary windings are not electrically coupled.
Referring to FIG. 1, a non-isolated transformer 10, on the other hand, includes primary 12 and series (secondary) 14', 41" windings which form a part of the same electrical circuit. In a step up configuration, the secondary winding 15 includes both the primary winding 12 and the additional series windings 14', 14". In operation, when alternating current is passed through the primary winding 12 magnetic flux is generated in the transformer (not illustrated) which in turn generates current in series windings thus increasing the combined voltage across the secondary winding 15.
In FIG. 1, 12 DC volts are provided to a center point 17 on the transformer 10 via input line 16 which is connected to a 12 volt automobile battery. A switching mechanism 18 alternately connects nodes 21 and 22 to ground via lines 19, 20 respectively so that the voltage at node 17 is alternately impressed across a first 12' and then a second 12" of the primary windings 12. When 12 volts is impressed across the first primary winding 12', the second primary 12" and second series 14" windings operate as a secondary thus providing +36 volts at node 21 which in turn forward biases diode 22 so that node 23 is also at +36 volts.
Similarly, when 12 volts is impressed across the second primary 12" (and node 22 is grounded), the first primary 12' and first series 14" windings operate as a secondary providing +36 volts at node 24 which forward biases diode 25 so that node 23 is again at +36 volts. Small filtering capacitors 26, 27 are provided at the front and back ends of the circuit for filtering purposes.
Generally, the non-isolated primary and secondary transformer is advantageous for the same reasons that an isolated winding transformer is disadvantageous and visa versa. For example, because the non-isolated transformer's secondary winding includes the primary winding, the combined number of windings to step voltage up a specific quantum is typically less with the non-isolated transformer, as opposed to an isolated transformer. In addition, because less windings are required with a non-isolated transformer, relatively less power is lost in the windings and usually a smaller transformer core can be used. As a corollary, given a specific core size with a non-isolated transformer winding configuration, greater core surface area can be exposed for heat dissipating purposes.
Despite all of the above mentioned advantages associated with-a non-isolated transformer, isolated non-series transformers are still preferred for many applications. One reason isolated transformers are preferred is their inherent ability to provide an electrical "open circuit" between their primary and secondary windings. This open circuit eliminates ground loop currents which cannot easily be eliminated using a non-isolated transformer alone.
Ground currents are caused when two or more points in an electrical system that are nominally at ground potential are connected by a conducting path such that the two point are not at the same potential. When this happens, a current is caused to flow between the two nominal ground points. In an audio system including an amplifier, ground loop potential can turn into undesirable ground loop noise and distort an audio signal as the amplifier amplifies the noise. A non-isolated transformer does not electrically separate primary and secondary coils and therefore, provides a ground loop circuit which can result in ground loop noise.
In an automobile power amplifier, ground loop potential can be a significant problem. In an automobile, the automobile chassis is typically connected to battery ground so that any component connected to the chassis is theoretically connected to a unipotential ground point. In reality, however, because automobile stereo components will typically be connected to ground through special ground wires characterized by different lengths and resistances, the separate grounding points may have slightly different potentials. Thus, ground loop potential problems are common in automobiles. For this reason, despite their size and associated costs, isolated transformers have been the standard in the automotive amplifier industry.
Recently, however, the industry has approached the ground loop potential problem from a different perspective. Instead of eliminating ground loop currents, the potential has been affirmatively compensated so that, in fact, differences in ground potential are rejected by the amplifier. Methods of affirmatively compensating for ground loop potential currently exist in the field and are well known.
Once a system has been designed wherein ground loop potential has been accounted for and proper system gain is achieved, it is necessary to obtain bipolar supply rails so as to allow straight forward implementation of a direct coupled power amplifier. Where isolated transformers have been used in the automobile audio industry, bipolar output has been achieved by employing a center tap output configuration including a full wave bridge rectifier. This type of configuration is well known in the industry.
However, the industry has yet to achieve bipolar output from a non-isolated transformer in an acceptable manner. Referring again to FIG. 1, typical automotive series connected non-isolated transformers provide a stepped up unipolar output and convert that unipolar output to bipolar output by passing the unipolar output through a large capacitor stage 30. While this solution provides ineffective bipolar rails, one of the advantages of the non-isolated transformer, its smaller relative size, is mitigated as the capacitor stage typically includes large capacitor components. Large capacitors are especially important for adequate low frequency response. In addition, even where large capacitors are used, low frequency response will typically be only acceptable at best.
Other output stages capable of providing bipolar output (with a unipolar supply) having acceptable low frequency response have been developed. However, these other output stages generally require additional hardware which increases amplifier costs considerably.
In light of the problems associated with providing positive and negative rails using a non-isolated transformer, the industry has opted to use isolated primary and secondary transformers so as to achieve bipolar rails and therefore good low frequency response without a costly output stage, this at the expense of increased amplifier size and increased transformer costs.
Thus, it would be advantageous to have a transformer stage for use with a direct coupled automotive audio amplifier which takes advantage of the benefits associated with a non-isolated series connected bipolar transformer (i.e. higher output/size ratio and relatively low cost) which can provide a bipolar output having superb low frequency response without a large or costly output stage.